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Previously, when LockProcessIfNeeded() was called for SiteInstances
without a site, it only ensured that the resulting process has an
allow-any-site lock.  However, when LockProcessIfNeeded() is called on
the same process more than once, such as when it gets reused by
different BrowsingInstances with blank SiteInstances, this is
insufficient.  We also need to call IncludeIsolationContext()
to inform ChildProcessSecurityPolicy of new BrowsingInstances.

This is required for relanding https://crrev.com/c/2446370, where
ChildProcessSecurityPolicy will start keeping track of a list of
BrowsingInstances for each process, and not doing this would lead to
mismatched process locks and crashes.

Bug: 1135539
Change-Id: Ie7b5320cb5a2101d9685169cf6eba254178165fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523732
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825510}